Semiconductor structure

ABSTRACT

A semiconductor structure is provided. A second area is disposed between first and third areas. An epitaxial layer is on a substrate. A body layer is in the epitaxial layer in first and second areas. First and second gates are in the body layer and in a portion of the epitaxial layer. The first gate is in the substrate and partially in first and second areas. The second gate is in the substrate and partially in second and third areas. A first contact plug is in a portion of the body layer in the first area. A second contact plug is at least in the epitaxial layer in the third area and contacts the epitaxial layer and the second gate. The first contact plug is electrically connected to the second contact plug. A first doped region is in the body layer between the first contact plug and the first gate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 12/542,687, filed on Aug. 17, 2009, now pending. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor structure and a methodof forming the same, and more particularly to a semiconductor structureintegrating a power metal-oxide-semiconductor field effect transistor(power MOSFET) and a Schottky diode and a method of forming the same.

2. Description of Related Art

Power metal-oxide-semiconductor field effect transistors (power MOSFETs)are widely applied to switching devices such as power supplies,rectifiers, low-voltage motor controllers and the like. FIG. 1illustrates a schematic cross-sectional view of a conventional powerMOSFET. Referring to FIG. 1, an N-type epitaxial layer 12 is disposed onan N-type heavily-doped substrate 10. A gate 16 is disposed in theN-type epitaxial layer 12. A P-type body layer 14 is disposed in theN-type epitaxial layer 12 beside the gate 16. An N-type heavily-dopedregion 18 is disposed in the P-type body layer 14 beside the gate 16. Adielectric layer 20 is disposed on the gate 16 and the N-typeheavily-doped region 18. A source metal layer 22 is disposed on thedielectric layer 20 and electrically connected to the N-typeheavily-doped region 18. A drain metal layer 24 is disposed on the otherside of the N-type heavily-doped substrate 10.

Along with an increasing demand for notebook computers and portableproducts, designs for sync-FETs having low output voltage, low forwardvoltage drop, low power loss and fast reverse recovery are required.However, due to the inherent PN diode between the P-type body layer 14and the N-type epitaxial layer 12, the aforementioned requirements arehard to satisfy.

One known method is to integrate a power MOSFET with a Schottky diode tomeet the above requirements. The current technology includes two types:silicon-in-one-package (SiP) and system-in-one-chip (SOC). The SiPplaces a power MOSFET in parallel with a Schottky diode. The formationprocess is simple, but the bonding wire connecting the power MOSFET andthe Schottky diode produces parastitic inductances, so as to limit theoverall efficiency. Although the SOC solves the parastitic inductances,the cell pitch thereof is relatively high (greater than 2 μm), so thatthe cell density can not be enhanced.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a semiconductor structurecapable of integrating a power MOSFET and a Schottky diode effectively,so as to enhance the cell density thereof.

The present invention further provides a method of forming asemiconductor structure. The process thereof is simple and can beintegrated with current processes.

The present invention provides a semiconductor structure including asubstrate of a first conductivity type, an epitaxial layer of the firstconductivity type, a body layer of a second conductivity type, a firstgate, a second gate, a first contact plug, a second contact plug and afirst doped region of the first conductivity type. The substrate has afirst area, a second area and a third area. The second area is disposedbetween the first area and the third area. The epitaxial layer isdisposed on the substrate. The body layer is disposed in the epitaxiallayer in the first area and the second area. The first gate and thesecond gate are disposed in the body layer and in a portion of theepitaxial layer outside the body layer. The first gate is disposed inthe substrate, partially in the first area and partially in the secondarea, and the second gate is disposed in the substrate, partially in thesecond area and partially in the third area. The first contact plug isdisposed in a portion of the body layer in the first area. The secondcontact plug is at least disposed in the epitaxial layer in the thirdarea and physically and electrically contacts the epitaxial layer andthe second gate. The first contact plug is electrically connected to thesecond contact plug. The first doped region is disposed in the bodylayer between the first contact plug and the first gate.

According to an embodiment of the present invention, the second contactplug is embedded into a second gate.

According to an embodiment of the present invention, the second contactplug is disposed in the epitaxial layer in the third area and a portionof the second area. Moreover, the second gate is disposed under thesecond contact plug.

According to an embodiment of the present invention, the body layer inthe second area covers a sidewall and a portion of a bottom of thesecond contact plug.

According to an embodiment of the present invention, the semiconductorstructure further includes a second doped region of the secondconductivity type, and the second doped region is disposed around abottom and a portion of a sidewall of the first contact plug.

According to an embodiment of the present invention, the semiconductorstructure further includes a dielectric layer and a metal layer. Thedielectric layer is disposed on the substrate and exposes the firstcontact plug and the second contact plug. The metal layer is disposed onthe substrate and covers the dielectric layer, the first contact plugand the second contact plug.

According to an embodiment of the present invention, a material of themetal layer includes AlSiCu, for example.

According to an embodiment of the present invention, the first contactplug and the second contact plug each include a first metal layer and abarrier layer around the first metal layer.

According to an embodiment of the present invention, a material of thefirst metal layer includes tungsten, for example.

According to an embodiment of the present invention, the firstconductivity type is N-type and the second conductivity type is P-type.Alternatively, the first conductivity type is P-type and the secondconductivity type is N-type.

According to an embodiment of the present invention, a cell pitch of thesemiconductor structure is about 1.5 μm or smaller.

According to an embodiment of the present invention, a cell density ofthe semiconductor structure is about 300 M cell/inch² or greater.

According to an embodiment of the present invention, the second gate isat the same potential with the metal layer.

According to an embodiment of the present invention, the first areasurrounds the second area and the second area surrounds the third area.

According to an embodiment of the present invention, the semiconductorstructure further includes at least one third gate, and the third gateis disposed in the epitaxial layer in the third area and located underthe second contact plug.

According to an embodiment of the present invention, the third gate isat the same potential with the metal layer.

In light of the foregoing, in the semiconductor structure of the presentinvention, since the tungsten contact plugs are disposed in the powerMOSFET area and the Schottky diode area, the cell pitch (a distancebetween adjacent power MOSFETs) is reduced to about 1.5 μm or smallerand the cell density is increased to about 300 M cell/inch² or higher.Furthermore, the method of the present invention is simple, easy, andcompatible with current formation processes and is thus a competitivemethod.

In order to make the aforementioned and other features and advantages ofthe invention more comprehensible, several embodiments accompanied withfigures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 illustrates a schematic cross-sectional view of a conventionalpower MOSFET.

FIG. 2 illustrates a schematic top view of a semiconductor structureaccording to a first embodiment of the present invention; herein, thetopmost metal layer and dielectric layer are omitted.

FIG. 2A is a schematic cross-sectional view taken along a line I-I′ inFIG. 2.

FIG. 3 illustrates a schematic top view of a modification of thesemiconductor structure according to the first embodiment of the presentinvention; herein, the topmost metal layer and dielectric layer areomitted.

FIG. 3A is a schematic cross-sectional view taken along a line I-I′ inFIG. 3.

FIG. 4 illustrates a schematic top view of another modification of thesemiconductor structure according to the first embodiment of the presentinvention; herein, the topmost metal layer and dielectric layer areomitted.

FIG. 4A is a schematic cross-sectional view taken along a line I-I′ inFIG. 4.

FIG. 5 illustrates a schematic top view of yet another modification ofthe semiconductor structure according to the first embodiment of thepresent invention; herein, the topmost metal layer and dielectric layerare omitted.

FIG. 5A is a schematic cross-sectional view taken along a line I-I′ inFIG. 5.

FIG. 6 illustrates a schematic top view of a semiconductor structureaccording to a second embodiment of the present invention; herein, thetopmost metal layer and dielectric layer are omitted.

FIG. 6A is a schematic cross-sectional view taken along a line I-I′ inFIG. 6.

FIGS. 7A through 7E are cross-sectional views schematically depicting amethod of forming a semiconductor structure according to the firstembodiment of the present invention.

FIGS. 8A through 8B are cross-sectional views schematically depicting amethod of forming a semiconductor structure according to the secondembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a schematic top view of a semiconductor structureaccording to a first embodiment of the present invention. Here, thetopmost metal layer and dielectric layer are omitted.

Referring to FIG. 2, in the semiconductor structure of the presentinvention, a floating diode surrounds the periphery of a Schottky diodeand sixteen power MOSFETs surround the periphery of the floating diode.The numbers of the Schottky diodes, the floating diodes and the powerMOSFETs are not limited by the present invention and are adjusted uponthe process or design requirements. Generally, the number of theSchottky diodes occupies about a tenth to a thirtieth of the number ofthe power MOSFETs.

FIG. 2A is a schematic cross-sectional view taken along a line I-I′ inFIG. 2. Referring to FIG. 2A, the semiconductor structure of the presentinvention includes a substrate 100 of a first conductivity type. Thesubstrate 100, for example, is an N-type heavily-doped (N+) siliconsubstrate. This N-type heavily-doped silicon substrate serves as a drainof the power MOSFET. The substrate 100 has a first area 102 a, a secondarea 102 b and a third area 102 c. The second area 102 b is disposedbetween the first area 102 a and the third area 102 c. As shown in thetop view of FIG. 2, the first area 102 a surrounds the second area 102 band the second area 102 b surrounds the third area 102 c. Accordingly,the cross-sectional view taken along a line I-I′ in FIG. 2 shows twopieces of the first area 102 a which are connected to each other(regards as “one” first area hereinafter), two pieces of the second area102 b which are connected to each other (regards as “one” second areahereinafter) and one third area 102 c, as shown in FIG. 2A. The firstarea 102 a is used for forming the power MOSFETs, the second area 102 bis used for forming the floating diode, and the third area 102 c is usedfor forming the Schottky diodes.

The semiconductor structure of the present invention further includes anepitaxial layer 104 of the first conductivity type, a body layer 106 ofa second conductivity type, a first gate 108, a second gate 110, firstcontact plugs 114, a second contact plug 116 and first doped regions 118of the first conductivity type.

The epitaxial layer 104 is disposed on the substrate 100. The epitaxiallayer 104 is an N-type lightly-doped (N−) epitaxial layer, for example.The body layer 106 is disposed in the epitaxial layer 104 in the firstarea 102 a and the second area 102 b. The body layer 106, for instance,is a P-type body layer.

The first gate 108 and the second gate 110 are disposed in the bodylayer 106 and in a portion of the epitaxial layer 104 outside the bodylayer 106. The first gate 108 is disposed partially in the first area102 a and partially in the second area 102 b. The second gate 110 isdisposed partially in the second area 102 b and partially in the thirdarea 102 c. As shown in FIG. 2, the second gate 110 is at the samepotential with the metal layer 120 and shaped as a rectangular ringalong the boundary between the second area 102 b and the third area 102c. Accordingly, the cross-sectional view taken along a line I-I′ in FIG.2 shows two pieces of the second gate 108 which are connected to eachother (regarded as “one” second gate hereinafter), as shown in FIG. 2A.Similarly, two pieces of the first gate 108 in FIG. 2A are connected toeach other and regarded as “one” first gate hereinafter. Furthermore,the first gate 108 and the second gate 110 each include a polysiliconlayer 109 and an oxide layer 107 around the polysilicon layer 109.

The first contact plugs 114 are disposed in a portion of the body layer106 in the first area 102 a. The second contact plug 116 is at leastdisposed in the epitaxial layer 104 in the third area 102 c andcontacting the epitaxial layer 104 and the second gate 110. In thepresent embodiment, the second contact plug 116 is disposed in theepitaxial layer 104 in the third area 102 c and embedded into the secondgate 110, as shown in FIG. 2A. In other words, the second gate 110covers a sidewall and a portion of a bottom of the second contact plug116. Moreover, the first contact plugs 114 and the second contact plug116 each include a metal layer 115 and a barrier layer 113 around themetal layer 115. A material of the metal layer 115 is, for example,tungsten, and a material of the barrier layer 113 is titanium ortitanium nitride, for instance. Each first doped region 118 is disposedin the body layer 106 between each first contact plug 114 and the firstgate 108. The first doped regions 118 are N-type heavily-doped regions,for example. Each N-type heavily-doped region serves as the drain of thepower MOSFET.

The semiconductor structure of the present invention further includes adielectric layer 112, a metal layer 120 and second doped regions 122 ofthe second conductivity type. The dielectric layer 112 is disposed onthe substrate 100 to expose the first contact plugs 114 and the secondcontact plug 116. The metal layer 120 is disposed on the substrate 100and covers the dielectric layer 112, the first contact plugs 114 and thesecond contact plug 116. That is, the first contact plugs 114 and thesecond contact plug 116 are electrically connected via the metal layer120. A material of the metal layer 120 is AlSiCu, for example. Eachsecond doped region 122 is disposed on a bottom and a portion of asidewall of each first contact plug 114. The second doped regions 122,for instance, are P-type heavily-doped (P+) doped regions to furtherreduce the resistance between the first contact plugs 114 and the bodylayer 106.

In the present embodiment, the first area 102 a is for forming powerMOSFETs. In the second area 102 b, the second gate 110 is at the samepotential with the metal layer 120, and the metal layer 120 is not incontact with the body layer 106. Therefore, the junction between theP-type body layer 106 and the N-type epitaxial layer 104 is a floatingdiode. In the third area 102 c, since the epitaxial layer 104 is anN-type lightly-doped epitaxial layer, the junction between the secondcontact plug 116 and the N-type epitaxial layer 104 is a Schottkycontact.

In the first embodiment, the third area 102 c merely includes oneSchottky diode as an illustration. However, the present invention is notlimited thereto. In the following, multiple modifications of the firstembodiment in which the third area 102 c includes a plurality ofSchottky diodes are illustrated.

FIG. 3 illustrates a schematic top view of a modification of thesemiconductor structure according to the first embodiment of the presentinvention; herein, the topmost metal layer and dielectric layer areomitted. FIG. 3A is a schematic cross-sectional view taken along a lineI-I′ in FIG. 3.

Referring to FIG. 3, the semiconductor structure of the presentinvention further includes a third gate 111 shaped as a rectangularring. Accordingly, the cross-sectional taken along a line I-I′ in FIG. 3shows two pieces of the third gate 111 which are connected to each other(regards as “one” third gate hereinafter), as shown in FIG. 3A. Thethird gate 111 is disposed in the epitaxial layer 104 in the third area102 c and located under the second contact plug 116. The second gate 110shaped as a rectangular ring surrounds the periphery of the third gate111, and the third gate 111 and the second gate 110 are all at the samepotential with the metal layer 120. In this modified embodiment, thethird area 102 c includes two Schottky diodes. In addition, a floatingdiode surrounds the periphery of the two Schottky diodes and sixteenpower MOSFETs surround the periphery of the floating diode.

FIG. 4 illustrates a schematic top view of another modification of thesemiconductor structure according to the first embodiment of the presentinvention; herein, the topmost metal layer and dielectric layer areomitted. FIG. 4A is a schematic cross-sectional view taken along a lineI-I′ in FIG. 4.

Referring to FIG. 4, the semiconductor structure of the presentinvention further includes a third gate 111 formed as a snake shape.Accordingly, the cross-sectional taken along a line I-I′ in FIG. 4 showsthree pieces of the third gate 111 which are connected to each other(regards as “one” third gate hereinafter), as shown in FIG. 4A. Thethird gate 111 is disposed in the epitaxial layer 104 in the third area102 c and located under the second contact plug 116. The third gate 111are connected to the second gate 110 arranged in stripes, and the thirdgate 111 and the second gate 110 are all at the same potential with themetal layer 120. In this modified embodiment, the third area 102 cincludes four Schottky diodes. In addition, a floating diode surroundsthe periphery of the four Schottky diodes and sixteen power MOSFETssurround the periphery of the floating diode.

FIG. 5 illustrates a schematic top view of yet another modification ofthe semiconductor structure according to the first embodiment of thepresent invention; herein, the topmost metal layer and dielectric layerare omitted. FIG. 5A is a schematic cross-sectional view taken along aline I-I′ in FIG. 5.

As shown in FIG. 5, the semiconductor structure of the present inventionfurther includes a third gate 111 substantially formed as a fishboneshape. As shown in FIG. 5A, the third gate 111 are disposed in theepitaxial layer 104 in the third area 102 c and located under the secondcontact plug 116. The third gate 111 are connected to the second gate110 shaped as a frame ring, and the third gate 111 and the second gate110 are all at the same potential with the metal layer 120. In thismodified embodiment, the third area 102 c includes eight Schottkydiodes. In addition, a floating diode surrounds the periphery of theeight Schottky diodes and fourteen power MOSFETs surround the peripheryof the floating diode.

FIG. 6 illustrates a schematic top view of a semiconductor structureaccording to a second embodiment of the present invention. Herein, thetopmost metal layer and dielectric layer are omitted. FIG. 6A is aschematic cross-sectional view taken along a line I-I′ in FIG. 6. Thesecond embodiment is similar to the first embodiment. Nevertheless, thedifference is that in the second embodiment, the second area 102 b isfor forming a body diode instead of the floating diode in the firstembodiment. The illustration on the difference between the first and asecond embodiments is described below, and the details are not iteratedherein.

In the second embodiment, the dispositions of the dielectric layer 112,the second contact plug 116 and the metal layer 120 are different, sothat a body diode is formed in the second area 102 b, which is differentfrom the technical feature of the floating diode forming in the secondarea 102 b in the first embodiment. In details, the second contact plug116 is disposed in the epitaxial layer 104 in the third area 102 c and aportion of the second area 102 b. Moreover, the second gate 110 isdisposed under the second contact plug 116 as shown in FIG. 6A.Additionally, the body layer 106 in the second area 102 b covers asidewall and a portion of a bottom of the second contact plug 116. Thedielectric layer 112 is disposed on the substrate 100 to expose thefirst contact plugs 114 and the second contact plug 116. The metal layer120 is disposed on the substrate 100 and covers the dielectric layer112, the first contact plugs 114 and the second contact plug 116.

In the present embodiment, the first area 102 a is for forming powerMOSFETs. Since the second contact plug 116 in the second area 102 b iselectrically connected to the metal layer 120, the junction between theP-type body layer 106 and the N-type epitaxial layer 104 is a bodydiode. In addition, the junction between the second contact plug 116 andthe N-type epitaxial layer 104 in the third area 102 c is a Schottkycontact.

In the second embodiment, the third area 102 c merely includes oneSchottky diode as an illustration. However, the present invention is notlimited thereto. Obviously, the third area 102 c can also include aplurality of Schottky diodes and the modifications and improvementsthereof can be referred to FIGS. 3-5 and FIGS. 3A-5A, and are thus notrepeated herein.

In light of the foregoing, the semiconductor structure of the presentinvention includes a substrate, at least one power MOSFET, a floatingdiode or a body diode and at least one Schottky diode. The substrate hasa first area, a second area and the third area. The second area isdisposed between the first area and the third area. The at least onepower MOSFET is disposed in the first area. The floating diode or bodydiode is disposed in the second area. The at least one Schottky diode isdisposed in the third area. Furthermore, the contact plugs of the powerMOSFET and the Schottky diode include tungsten and are electricallyconnected to each other.

In the semiconductor structure of the present invention, since thetungsten contact plugs are disposed in the power MOSFET area and theSchottky diode area, the cell pitch (a distance between adjacent powerMOSFETs) is reduced from 2.0 μm to about 1.5 μm or smaller and the celldensity is increased to 300 M cell/inch² or higher. Compared with theconventional SOP structure, the semiconductor structure of the presentinvention not only integrates the power MOSFET and the Schottky diodeeffectively, but also enhances the cell density, thereby greatlyenhancing the competitiveness thereof.

A method of forming the semiconductor structure of the present inventionis explained hereinafter. FIGS. 7A through 7E are cross-sectional viewsschematically depicting a method of forming a semiconductor structureaccording to the first embodiment of the present invention.

Firstly, referring to FIG. 7A, an epitaxial layer 104 of a firstconductivity type is formed on a substrate 100 of the first conductivitytype. The substrate 100, for example, is an N-type heavily-doped siliconsubstrate. The substrate 100 serves as a drain of the power MOSFET. Thesubstrate 100 has two pieces of a first area 102 a (regarded as “one”first area hereinafter), two pieces of a second area 102 b (regarded as“one” second area hereafter) and a third area 102 c. The second area 102b is disposed between the first area 102 a and the third area 102 c. Inthe present embodiment, the third area 102 c is disposed in the center,and two pieces of the second area 102 b and two pieces of the first area102 a are respectively disposed mirror-symmetrically to the third area102 c. The epitaxial layer 104, for example, is an N-type lightly-dopedepitaxial layer, and a formation method thereof includes an epitaxialgrowth process.

Next, a first gate 108 and a second gate 110 are formed in the epitaxiallayer 104. The first gate 108 is disposed partially in the first area102 a and partially in the second area 102 b. The second gate 110 isdisposed partially in the second area 102 b and partially in the thirdarea 102 c. A method of forming the first gate 108 and the second gate110 includes the following steps. First of all, trenches for forming thefirst gate 108 and the second gate 110 are etched in the epitaxial layer104. Thereafter, an oxide layer 107 is conformally formed on thesubstrate 100 and surfaces of the trenches. A material used for formingthe oxide layer 107 is, for example, silicon dioxide, and the method offorming the same is, for example, a thermal oxidation process.Afterwards, a polysilicon layer 109 is filled in the trenches. A methodof forming the polysilicon layer 109 includes a chemical vapordeposition (CVD) process. During the step of forming the first gate 108and the second gate 110 in the epitaxial layer 104, at least one thirdgate (not shown) is further formed in the epitaxial layer 104 if thefinal structure as shown in FIGS. 3-5 and FIGS. 3A-5A are required toform.

Referring to FIG. 7B, a patterned photoresist layer 105 is formed on thesubstrate 100. After that, an ion implantation process is performed byusing the patterned photoresist layer 105 as a mask, so as to form abody layer 106 of a second conductivity type in the epitaxial layer 104in the first area 102 a and the second area 102 b. The body layer 106,for instance, is a P-type body layer. Subsequently, the patternedphotoresist layer 105 is removed and a drive-in process is performed. Inthis step, the third area is covered by the patterned photoresist layer105, so as to avoid an undesired PN junction formed in the third area102 c, which is beneficial for the subsequent formation of the Schottkydiode.

Referring to FIG. 7C, first doped regions 118 of the first conductivitytype are formed in the body layer 106 in the first area 102 a. The firstdoped regions 118 are N-type heavily-doped regions. Each N-typeheavily-doped region serves as a source of the power MOSFET. A method offorming the first doped regions 118 includes an ion implantation processand the subsequent drive-in process. Next, a dielectric material layer(not shown) and a patterned photoresist layer 117 are sequentiallyformed on the substrate 100. Thereafter, a portion of the dielectricmaterial layer and a portion of the oxide layer 107 are removed by usingthe patterned photoresist layer 117 as a mask, so as to form adielectric layer 112. The dielectric layer 112 exposes a portion of thefirst doped regions 118 in the first area 102 a, at least the epitaxiallayer 104 in the third area 102 c and at least a portion of the secondgate 110. In the present embodiment, the dielectric layer 112 exposes aportion of the first doped regions 118 in the first area 102 a, theepitaxial layer 104 in the third area 102 b and a portion of the secondgate 110.

Referring to FIG. 7D, a portion of the body layer 106, a portion of thefirst doped regions 118, a portion of the epitaxial layer 104 and aportion of the second gate 110 are removed by using the dielectric layer112 as a mask. Therefore, first openings 124 are formed in the firstdoped regions 118 a and in a portion of the body layer 106 outside thefirst doped regions 118 in the first area 102 a. Additionally, a secondopening 126 is formed in the epitaxial layer 104 in the third area 102 cand in a portion of the second gate 110 a. Afterwards, a patternedphotoresist layer 121 is formed on the substrate 100. The patternedphotoresist layer serves as a mask for carrying out an ion implantationprocess so as to form second doped regions 122 of the secondconductivity type in bottoms of the first openings 124. The second dopedregions 122 are P-type heavily-doped regions, for example. In this step,the same mask for forming the body layer 106 (refer to FIG. 7B) is usedfor forming the second doped regions 122. In other words, the same bodylayer mask can be used twice, and no additional masks are required tocomplete the second doped regions 122. In this step, not only are thesecond doped regions 122 formed to further reduce the resistance betweenthe body layer 106 and the subsequently formed first contact plugs 114,but the patterned photoresist layer 121 covering the third area 102 cprevents the formation of an undesired PN junction in the third area 102c, which is beneficial for the subsequent formation of the Schottkydiode.

Referring to FIG. 7E, the patterned photoresist layer 121 is removed andthe drive-in process is performed. In this step, high temperature causesthe second doped regions 122 to diffuse to a periphery thereof, therebycovering a portion of sidewalls of the first openings 124. Thereafter, abarrier layer 113 and a metal layer 115 are sequentially filled in thefirst openings 124 and the second opening 126. A material of the barrierlayer 113 is, for example, titanium or titanium oxide, and a material ofthe metal layer 115 is tungsten, for instance. A method of forming thebarrier layer 113 and the metal layer 115 includes a sputtering or CVDprocess. The barrier layer 113 and the metal layer 115 in the firstopenings 124 form first contact plugs 114. The barrier layer 113 and themetal layer 115 in the second opening 126 form a second contact plug116. Thereafter, a metal layer 120 is formed on the substrate 100 tocover the dielectric layer 112, the barrier layer 113 and the metallayer 115. Up to this point, the formation of the power MOSFETs in thefirst area 102 a, the floating diode in the first area 102 a and theSchottky diode in the third area 102 c are completed.

FIGS. 8A to 8B are cross-sectional views schematically depicting amethod of forming a semiconductor structure according to the secondembodiment of the present invention. The difference between the secondembodiment and the first embodiment is that in the second embodiment,the second area 102 b is for forming a body diode instead of thefloating diode in the first embodiment. The differences of the firstembodiment and the second embodiment are illustrated in the following,and the details are not iterated herein.

First of all, an intermediate structure as shown in FIG. 7B is provided.Next, the patterned photoresist layer 105 is removed. Referring to FIG.8A, first doped regions 118 of the first conductivity type are formed inthe body layer 106 in the first area 102 a. The first doped regions 118are the N-type heavily-doped regions, for example. Next, the dielectricmaterial layer (not shown) and the patterned photoresist layer 117 aresequentially formed on the substrate 100. Thereafter, a portion of thedielectric material layer and a portion of the oxide layer 107 areremoved by using the patterned photoresist layer 117 as a mask, so as toform a dielectric layer 112. The dielectric layer 112 exposes a portionof the first doped regions 118 in the first area 102 a, the epitaxiallayer 104 in the third area 102 c, the entire second gate 110, and aportion of the body layer 106 in the second area 102 b.

Referring to FIG. 8B, a portion of the body layer 106, a portion of thefirst doped regions 118, a portion of the epitaxial layer 104 and aportion of the second gate 110 are removed by using the dielectric layer112 as a mask. Therefore, first openings 124 are formed in the firstdoped regions 118 and a portion of the body layer 106 outside the firstdoped regions 118 in the first area 102 a. Additionally, a secondopening 126 is formed in the epitaxial layer 104 in the third area 102 cand a portion of the second area 102 b. After that, second doped regions122 of the second conductivity type are formed in bottoms of the firstopenings 124. The second doped regions 122 are the P-type heavily-dopedregions, for example. Thereafter, a barrier layer 113 and a metal layer115 are sequentially filled in the first openings 124 and the secondopening 126. The metal layer 120 is then formed on the substrate 100 tocover the dielectric layer 112, the barrier layer 113 and the metallayer 115.

In the embodiments illustrated, the first conductivity type is N-typeand the second conductivity type is P-type. However, the presentinvention is not limited thereto. People skilled in the art shouldunderstand that the first conductivity type can also be P-type while thesecond conductivity type is N-type.

In summary, in the semiconductor structure of the present invention, atleast one Schottky diode is disposed inside a plurality of powerMOSFETs. Moreover, the power MOSFETs and the Schottky diode areseparated by the floating diode or body diode. By disposing the tungstencontact plugs that are electrically connected to one another in thepower MOSFET area and the Schottky area, not only are the power MOSFETand the Schottky diode integrated effectively, but the cell pitch (adistance between adjacent power MOSFETs) can be reduced from 2.0 μm toabout 1.5 μm or smaller. Hence, the power MOSFET area and the Schottkyarea are integrated using the SOP technique, and the cell density isincreased to about 300 M cell/inch² or higher, thereby greatly enhancingthe competitiveness thereof. Moreover, the method in accordance with thepresent invention does not require additional masks to complete thesemiconductor structure of the present invention. The process is simpleand compatible with the conventional processes.

Although the present invention has been described with reference to theabove embodiments, it will be apparent to one of the ordinary skill inthe art that modifications to the described embodiment may be madewithout departing from the spirit of the invention. Accordingly, thescope of the invention will be defined by the attached claims not by theabove detailed descriptions.

1. A semiconductor structure, comprising: a substrate of a firstconductivity type, having a first area, a second area and a third area,wherein the second area is disposed between the first area and the thirdarea; an epitaxial layer of the first conductivity type, disposed on thesubstrate; a body layer of a second conductivity type, disposed in theepitaxial layer in the first area and the second area; a first gate anda second gate, disposed in the body layer and in a portion of theepitaxial layer outside the body layer, wherein the first gate isdisposed in the substrate, partially in the first area and partially inthe second area, and the second gate is disposed in the substrate,partially in the second area and partially in the third area; a firstcontact plug, disposed in a portion of the body layer in the first area;a second contact plug, at least disposed in the epitaxial layer in thethird area and physically and electrically contacting the epitaxiallayer and the second gate, wherein the first contact plug iselectrically connected to the second contact plug; and a first dopedregion of the first conductivity type, disposed in the body layerbetween the first contact plug and the first gate.
 2. The semiconductorstructure as claimed in claim 1, wherein the second contact plug isembedded into the second gate.
 3. The semiconductor structure as claimedin claim 1, wherein the second contact plug is disposed in the epitaxiallayer in the third area and a portion of the second area, and the secondgate is disposed under the second contact plug.
 4. The semiconductorstructure as claimed in claim 3, wherein the body layer in the secondarea covers a sidewall and a portion of a bottom of the second contactplug.
 5. The semiconductor structure as claimed in claim 1, furthercomprising a second doped region of the second conductivity typedisposed around a bottom and a portion of a sidewall of the firstcontact plug.
 6. The semiconductor structure as claimed in claim 1,further comprising: a dielectric layer, disposed on the substrate andexposing the first contact plug and the second contact plug; and a metallayer, disposed on the substrate and covering the dielectric layer, thefirst contact plug and the second contact plug.
 7. The semiconductorstructure as claimed in claim 6, wherein a material of the metal layercomprises AlSiCu.
 8. The semiconductor structure as claimed in claim 1,wherein the first contact plug and the second contact plug each comprisea first metal layer and a barrier layer around the first metal layer. 9.The semiconductor structure as claimed in claim 8, wherein a material ofthe first metal layer comprises tungsten.
 10. The semiconductorstructure as claimed in claim 1, wherein the first conductivity type isN-type while the second conductivity type is P-type; or the firstconductivity type is P-type while the second conductivity type isN-type.
 11. The semiconductor structure as claimed in claim 1, wherein acell pitch of the semiconductor structure is smaller than or equal toabout 1.5 μm.
 12. The semiconductor structure as claimed in claim 1,wherein a cell density of the semiconductor structure is greater than orequal to about 300 M cell/inch².
 13. The semiconductor structure asclaimed in claim 6, wherein the second gate is at the same potentialwith the metal layer.
 14. The semiconductor structure as claimed inclaim 1, wherein the first area surrounds the second area and the secondarea surrounds the third area.
 15. The semiconductor structure asclaimed in claim 6, further comprising at least one third gate disposedin the epitaxial layer in the third area and located under the secondcontact plug.
 16. The semiconductor structure as claimed in claim 15,wherein the third gate is at the same potential with the metal layer.